1. Field of the Invention
The present invention relates to a transfer device used for an electrophotographic process (image-forming system) such as in a copier, printer, facsimile, etc. More specifically, the invention relates to a transfer device which stably and reliably limits the gap between a transfer roller and a photosensitive material in order to form an excellently transferred image.
2. Description of the Prior Art
As a transfer device for transferring a toner image formed on a photosensitive material drum in an image-forming apparatus based on the electrophotographic process, there has been known the one which employs a transfer roller that faces the photosensitive material drum, in order to transfer the image while passing a transfer sheet such as paper or the like between the photosensitive material drum and the transfer roller.
The transfer device which uses the transfer roller has an advantage in that it produces ozone in small amounts as compared with a transfer device which uses a corona charger.
It has also been known to dispose the transfer roller separated away from the photosensitive material. Japanese Unexamined Patent Publication (Kokai) No. 308843/1994 discloses a transfer device in an image-forming apparatus which includes an image-carrier (photosensitive material drum) and a transfer roller, and passes a transfer sheet therebetween to transfer the toner of the image carrier onto the transfer sheet, wherein the image carrier is an organic photosensitive material containing a charge-generating agent and a charge-transporting agent, the transfer roller formed of an electrically conducting polyurethane rubber composition which is so cured that the rubber hardness exceeds 50 degrees (JIS A), and the image carrier and the transfer roller are disposed being separated away from each other maintaining a small gap that is larger than the thickness of the transfer sheet but permits the toner to be transferred onto the transfer sheet.
To dispose the transfer roller maintaining a predetermined gap with respect to the photosensitive material, a positioning roller is generally brought into contact with the surface of the photosensitive material or with an extension thereof using an urging means such as spring or the like, so that a predetermined gap (which may often be smaller than the thickness of the paper) is maintained between the transfer roller and the photosensitive material.
In the transfer system using such a transfer roller, a guide plate must be provided in position between the transfer roller and the photosensitive material drum so that the transfer sheet such as paper is fed into between the transfer roller and the photosensitive material drum without causing the paper to be jammed.
To determine the position of the guide plate, it can be contrived to rotatably support a guide unit equipped with a guide plate in concentric with the transfer roller, provide a pair of engaging protrusions at both ends of the guide plate to come into engagement with the stoppers on the side of the photosensitive material drum, and provide a plurality of guide plate-urging means (resilient members such as springs or the like) for pushing the engaging protrusions onto the stoppers.
In positioning the transfer roller and the guide plate with respect to the photosensitive material drum, however, the support is made at four points, i.e., a pair of press-contacting points of the positioning roller for limiting the gap of the transfer roller relative to the photosensitive material drum and another pair of press-contacting points of the engaging protrusions of the guide plate relative to the stoppers. In practically assembling the apparatus, however, it can never be expected that these points are positioned on the same plane.
The positioning relying on the three-point support is most stable. However, the four-point support becomes absolutely necessary for positioning the transfer roller and the guide plate with respect to the photosensitive material drum. Here, it is quite likely that one point among the four points may be deviated from a common plane, causing the positioning to loose stability
In particular, a problem stems from a pair of press-contacting points of the positioning roller for limiting the gap of the transfer roller with respect to the photosensitive material drum. When either one of these press-contacting points is deviated, the gap undergoes a change between the transfer roller and the photosensitive material drum, whereby the image density changes and the image is disturbed, making it difficult to transfer the image maintaining a predetermined quality.
This tendency becomes particularly conspicuous in the non-contacting transfer system in which the transfer roller and the photosensitive material drum are not contacted to each other.